The invention relates to a device for ultrasound measurement of a volume-related quantity of a fluid, which device makes it possible to limit the number of acoustic propagation modes, and to improve the linearity of its calibration curve.
It is widely known to use ultrasound transducers for measuring a volume-related quantity of a fluid flowing along a duct. Conventionally, the ultrasound transducers are spaced apart from each other in the fluid flow, defining between them an ultrasound measurement path, and they are disposed, for example, face to face and in alignment on the longitudinal flow direction, or else they are mounted in one of the walls of the duct.
Alternatively, the transducers emit and receive ultrasound signals propagating in the fluid flow and electronic means are provided for determining the volume-related quantity of fluid flowing along the duct on the basis of measurements of at least one physical magnitude that is characteristic of the propagation speed of ultrasound signals in the fluid flow. By way of example, one such physical magnitude is the propagation time of ultrasound signals, and another is the phase thereof.
It is also known that devices implementing this type of measurement are subject to measurement errors that show up as lack of accuracy in the measurement and as non-linearity of the calibration curve for such a device. These errors are largely attributable to ultrasound waves propagating in the fluid flow in numerous acoustic modes other than plane mode, thereby giving rise to interfering phase shifts.
International patent application WO 93/00570 proposes a method of reducing such errors based on a technique of transmitting wave packets, in which every four wave packets are inverted relative to the preceding packets so as to cancel the effects of higher propagation modes in the duct. Although that method is effective, it nevertheless suffers from the drawback of being complicated to manage from the points of view of the associated electronics and of signal processing, and it consumes a non-negligible amount of energy.
International patent application WO 94/09342 describes an ultrasound measurement device in which the two ultrasound transducers are spaced apart in the fluid flow direction from the inlet to the outlet of the device, and including means for measuring the propagation times of ultrasound signals between the two transducers. That device defines a plurality of fluid flow passages that are mutually parallel, with each passage being dimensioned so as to have a characteristic cutoff frequency for wave propagation in plane mode that is higher than the transmission frequency of the ultrasound, thereby ensuring that waves can propagate in plane mode only along said passage. Such a device does not consume additional energy but it does introduce further interfering phase shifts due to the presence of the plurality of passages, and the interfering phase shifts affect the linearity of the calibration curve.